1. Field of the Invention
The invention relates to the production of primary aluminum by molten salt electrolysis using a cermet anode, and a TiB.sub.2, or TiB.sub.2 -graphite cathode in a bipolar electrode configuration.
2. Description of the Prior Art
(a) The Hall-Heroult Cell
Aluminum is commercially produced by the electrolysis of alumina in molten cryolite using conductive carbon electrodes, with the overall reaction: ##STR1##
Typically the Hall cell is a shallow vessel, with the floor forming the cathode, the side walls a rammed coke-pitch mixture, and the anode a block suspended in the molten cryolite bath at an anode-cathode separation of a few centimeters. The anode is formed from a pitch-calcined petroleum coke blend, prebaked to form a monolithic block of amorphous carbon. The cathode is typically formed from a prebaked pitch-calcined anthracite or coke blend, with cast-in-place iron over steel bar electrical conductors in grooves in the bottom side of the cathode.
(b) The Anode
The problems caused by use of carbon anodes are related to the cost of the anode consumed in the above reaction and to the impurities introduced to the melt from the carbon source. The petroleum cokes used in the fabrication of the anodes generally have significant quantities of impurities, principally sulfur, silicon, vanadium, titanium, iron and nickel. Sulfur is oxidized to its oxides, causing troublesome workplace and environmental pollution problems. The metals, particularly vanadium, are undesirable as contaminants in the aluminum metal produced. Removal of excess quantities of the impurities requires extra and costly steps when high purity aluminum is to be produced.
Attempts have been made in the past to use non-consumable anodes with little apparent success. Metals either melt at the temperature of operation, or are attacked by oxygen and/or the cryolite bath. Ceramic compounds, such as oxides with perovskite and spinel crystal structures, usually have too high electrical resistance or are attacked by the cryolite bath.
Previous efforts in the field are disclosed in U.S. Pat. No. 3,718,550--Klein, Feb. 27, 1973, Cl. 204/67; U.S. Pat. No. 4,039,401--Yamada et al., Aug. 2, 1977, Cl. 204/67; U.S. Pat. No. 4,098,669--de Nora et al., July 4, 1978, Cl. 204/252; Belyaev+Studentsov, Legkie Metal 6, No. 3, 17-24 (1937), (C.A. 31 [1937], 8384) and Belyaev, Legkie Metal 7, No. 1, 7-20 (1938) (C.A. 32 [1938], 6553).
Of the above references, Klein discloses an anode of at least 80% SnO.sub.2, with additions of Fe.sub.2 O.sub.3, ZnO, Cr.sub.2 O.sub.3, Sb.sub.2 O.sub.3, Bi.sub.2 O.sub.3, V.sub.2 O.sub.5, Ta.sub.2 O.sub.5, Nb.sub.2 O.sub.5 or WO.sub.3. Yamada discloses spinel structure oxides of the general formula XYY'O.sub.4 and perovskite structure oxides of the general formula RMO.sub.3, including the compounds CoCr.sub.2 O.sub.4, TiFe.sub.2 O.sub.4, NiCr.sub.2 O.sub.4, NiCo.sub.2 O.sub.4, LaCrO.sub.3, and LaNiO.sub.3. Balyaev discloses anodes of Fe.sub.2 O.sub.3, SnO.sub.2, Co.sub.3 O.sub.4, NiO, ZnO, CuO, Cr.sub.2 O.sub.3 mixtures thereof as ferrites. De Nora discloses Y.sub.2 O.sub.3 with Y, Zr, Sn, Cr, Mo, Ta, W, Co, Ni, Fd, Ag, and oxides of Mn, Rh, Ir, and Ru.
Problems with the materials above are related to the poor corrosion resistance of the materials, the cost of the raw materials, the fragility of the electrodes, the difficulty of making a sufficiently large electrode for commercial usage, and the low electrical conductivity of many of the materials when compared to carbon anodes.
U.K. Patent application No. 2,069,529, published Aug. 26, 1981 (and related U.K. Patent application No. 2,078,259, published Jan. 6, 1982), discloses cermet anodes useful for electrowinning metals from fused salt baths, such as aluminum from fused cryolite-alumina, which are composed of a ceramic phase and a metallic phase selected from a limited number of oxides and metals. The ceramic phase includes oxides such as ferrites and chromites of manganese, iron, cobalt, nickel, copper and zinc, and the metallic phase is selected from the metals chromium, iron, cobalt, nickel, copper and noble metals. The amount of metal phase incorporated in these cermets varies from about 2% to 30% by volume, preferably 10% to 20%. Reference is also made to U.S. Pat. No. 4,397,729, issued Aug. 9, 1983 (filed Jan. 16, 1981) to Duruz et al.; U.S. Pat. No. 4,374,050, issued Feb. 15, 1983 (filed Nov. 10, 1980) to Ray; U.S. Pat. No. 4,374,761, issued Feb. 22, 1983 (filed Nov. 10, 1980) to Ray, which concern cermet anodes for electrowinning metals from fused salts; and Ser. No. 475,951, Secrist et al., discloses a cermet anode assembly; Ser. Nos. 491,089 and 554,068, Secrist et al., disclose a cermet anode; Ser. No. 540,885, Landon et al. discloses an anode composition; Ser. No. 559,723, Grindstaff et al., discloses a method of producing aluminum alloys using cermet anodes; Ser. No. 560,456, Secrist et al., discloses a cermet electrode assembly.
(c) The Cathode
During operation of the Hall cell, only about 25% of the electricity consumed is used for the actual reduction of alumina to aluminum, with approximately 40% of the energy consumed by the voltage drop across the bath. The anode-cathode spacing is usually about 4-5 cm., and attempts to lower this distance result in an electrical discharge from the cathode to the anode through aluminum droplets.
The molten aluminum is present as a pad in the cell, but is not a quiescent pool due to the factors of preferential wetting of the carbon cathode surface by the cryolite melt in relation to the molten aluminum, causing the aluminum to form droplets, and the erratic movements of the molten aluminum from the strong electromagnetic forces generated by the high current density.
Typically, amorphous carbon is a low energy surface, but also is quite durable, lasting for several years duration as a cathode, and relatively inexpensive. However, a cathode or a cathode component such as TiB.sub.2 stud which has better wettability would permit closer anode-cathode spacing.
It had previously been known that refractory hard metals (RHM) are useful as a cathode component in the electrolytic production of aluminum, when retrofitted in the Hall cell as a replacement for the carbon or semi-graphite form. If the anode-cathode (A-C) distance could be lowered, the % savings in electricity would be as follows:
______________________________________ A-C distance % savings ______________________________________ 3.8 cm. std. 1.9 cm. 20% 1.3 cm. 27% 1.0 cm. 30% ______________________________________
Refractory hard metals (RHM) as a class are hard, dense materials with high melting points, and are generally of low solubility and resistant to corrosive attack by most acids and alkalis. They also have high electrical conductivity due to their metallic structure; consequently, this combination of properties has made them important candidates for use as cathodes in molten salt electrolysis processes where their corrosion resistance and conductivity are vital properties needed for economical performance.
RHM articles have been produced by a number of processes including hot pressing of the granular or powdered materials, chemical vapor deposition, and in situ reduction of metals by carbon or other reducing agents. Hot pressing is the most commonly used process for the production of shapes. A die and cavity mold set is filled with powder, heated to about 300.degree.-800.degree. C., and placed under pressure of about 2.times.10.sup.8 Pa to produce a preform. The preform is then removed from the mold and heated at about 1500.degree.-2000.degree. C., or higher to increase density.
Hot pressing has the limitations of applicability to simple shapes only, erosion of the mold, and slow production. The pieces produced by hot pressing are subject to a high percentage of breakage in handling, making this process expensive in terms of yield of useful products.
The RHMs of most interest include the carbides, borides, and nitrides of the metals of Groups IVA, IVB, VB, and VIB of the periodic table, particularly Ti, V, Si, and W. Of these, the borides are of most interest as electrodes in high temperature electrolysis applications due to their electrical conductivity, and of the borides, TiB.sub.2 has been extensively investigated for use as a cathode or cathodic element in the Hall-Heroult cell.
Several workers in the field have developed refractory high free energy material cathodes. U.S. Pat. No. 2,915,442, Lewis, Dec. 1, 1959, claims a process for production of aluminum using a cathode consisting of the borides, carbides, and nitrides of Ti, Zr, V, Ta, Nb, and Hf. U.S. Pat. No. 3,028,324, Ransley, Apr. 3, 1962, claims a method of producing aluminum using a mixture of TiC and TiB.sub.2 as the cathode. U.S. Pat. No. 3,151,053, Lewis, Sept. 29, 1964, claims a Hall cell cathode conducting element consisting of one of the carbides and borides of Ti, Zr, Ta and Nb. U.S. Pat. No. 3,156,639, Kibby, Nov. 10, 1964, claims a cathode for a Hall cell with a cap of refractory hard metal and discloses TiB.sub.2 as the material of construction. U.S. Pat. No. 3,314,876, Ransley, Apr. 18, 1967, discloses the use of TiB.sub.2 for use in Hall cell electrodes. The raw materials must be of high purity particularly in regard to oxygen content, Col. 1, line 73-Col. 2, line 29; Col. 4, lines 39-50, Col. 8, lines 1-24. U.S. Pat. No. 3,400,061, Lewis, Sept. 3, 1968 discloses a cathode comprising a refractory hard metal and carbon, which may be formed in a one-step reaction during calcination. U.S. Pat. No. 4,071,420, Foster, Jan. 31, 1978, discloses a cell for the electrolysis of a metal component in a molten electrolyte using a cathode with refractory hard metal TiB.sub.2 tubular elements protruding into the electrolyte. Ser. No. 043,242, Kaplan et al. (Def. Pub.), filed May 29, 1979, discloses Hall cell bottoms of TiB.sub.2. EPA 042658 discloses RHM cathodic elements. The principal deterrent to the use of a RHM as a Hall cell cathode has been the sensitivity to thermal shock and the great cost, as compared to the traditional carbonaceous compositions. U.S. Pat. No. 4,376,029, Joo' et al., discloses TiB.sub.2 -graphite composites used as cathodes; also U.S. Pat. No. 4,377,463, Joo' et al.; U.S. Pat. No. 4,439,382, Joo' et al., and Ser. No. 287,129, Juel et al., co-pending.
(d) Bipolar Technology
The ultimate end of the developments above is the use of long-lasting or relatively permanent anode and cathode materials in bipolar electrodes in a modified Hall-Heroult cell specially designed to make maximum use of the permanence of both components and the wettability of the cathodic component to produce the most energy and labor efficient and non-polluting cell possible.
It is generally accepted that aluminum could be produced most efficiently in a Hall-Heroult type cell equipped with dimensionally stable bipolar electrodes. Such a cell, with the electrodes deployed in closely-spaced vertical or horizontal arrays, should operate with the lowest energy requirement and demand less capital outlay per unit of aluminum production due to the high electrode packing density.
Bipolar electrodes of various design and composition have been disclosed by several workers. U.S. Pat. No. 4,187,155, DeNora, Feb. 5, 1980, discloses an anode and a bipolar electrode comprised of an oxy-compound of at least one metal from the group of La, Tb, Er, Yb, Th, Ti, Zr, Hf, Nb, Cr and Ta, an electroconductive agent, and a surface catalyst.
U.S. Pat. No. 4,111,765, DeNora et al., Sept. 5, 1978, discloses sintered electrodes having 40-90% of valve metal boride, 5-40% of SiC, and 5-40% of C. A bipolar electrode using these materials is disclosed at column 5, lines 36-54. It has been the experience of the inventors that such refractory hard metals are rapidly attacked when used as anodes and are primarily useful as cathodic elements.
U.S. Pat. No. 3,930,967, Alder, discloses vertically propagated cells having an advantage of easy transport of metal to a single sump using the same channels provided for bath circulation. A major shortcoming of the bipolar assembly described is the unacceptable contact resistance observed for this configuration since the components are clamped together only by mechanical pressure.
U.S. Pat. No. 4,347,050, Ray, discloses a bipolar electrode having an anode comprising two oxides, e.g. NiO and Fe.sub.2 O.sub.3, a metal separator, e.g. Ni, or stainless steel, and a TiB.sub.2 cathode. U.S. Pat. No. 4,374,764, Ray, discloses a bipolar electrode composed of a ceramic anode and a carbon or TiB.sub.2 cathode separated by Ni, Fe or Cr alloys.
The major technical problems to be addressed in the development of a bipolar electrode are:
1. fabricating anode and cathode materials with dissimilar expansion coefficients into a monolithic structure which will exhibit low ohmic losses,
2. maintaining acceptable internal stability of the electrode during extended cell operation at 950.degree. C., and
3. protecting the perimeter of the anode/cathode interface from attack by melt constituents.